In water treatment design, the removal efficiency of a horizontal-flow sedimentation tank primarily depends on surface overflow rate (flow per unit plan area). Which of the following parameters does the efficiency not depend upon (i.e., has the least direct influence under ideal conditions)?

Introduction / Context:
Sedimentation (plain settling) is a fundamental unit process in water and wastewater treatment. For an ideal horizontal-flow clarifier, particle removal is controlled by the relationship between particle settling velocity and the basin’s surface overflow rate. This question checks whether you know which geometric or hydraulic parameters most strongly govern efficiency and which have only secondary or negligible influence in the ideal theory.

Given Data / Assumptions:

• Horizontal-flow sedimentation tank operating under steady conditions.
• Clarifier design follows the ideal settling basin theory.
• Overflow rate = Q / A_plan (flow divided by plan surface area).

Concept / Approach:
In the ideal model, a particle is removed if its settling velocity vs ≥ overflow rate (Q/A). Therefore, the fraction removed is governed by plan surface area, not by the tank’s depth. Length, velocity distribution, and detention time influence short-circuiting and flocculation opportunities in real tanks, but the first-order determinant in the ideal analysis is surface overflow rate.

Step-by-Step Solution:

Verification / Alternative check:
Design handbooks emphasize that enlarging plan area (length or width) reduces overflow rate and improves removal, while simply deepening a tank has little benefit for removal efficiency under ideal assumptions.

Why Other Options Are Wrong:

• Length of tank: affects hydraulics, distribution, and approximation to plug flow.
• Detention period: relates to flow and volume; while not the primary ideal parameter, it correlates with performance in practical designs.
• Velocity of water: affects turbulence and short-circuiting; too high velocity reduces performance.

Common Pitfalls:
Assuming that increasing depth always improves performance; confusing storage volume (which affects detention) with the surface-governed efficiency criterion.

Final Answer:
depth of tank